This project will determine the suitability of the earthworm Lumbricus terrestris as a non-controversial model for 1) screening environmental xenobiotics for their potential immunotoxicity and 2) analyzing the mechanisms by which xenobiotics can interfere with recognition, processing and effector phases of immunity. The earthworm possesses many components of an effective animal model for screening chemicals for immunotoxic effects: 1) it shares many immunological characteristics with higher animals, including humans; 2) its immunobiology is well-understood; 3) it has a history of use as a bioassay organism, particularly with biocides; 4) it is an extremely cost-effective experimental animal; and 5) being an invertebrate, it is a non-controversial experimental animal. Our preliminary work on the immunotoxicity of Arochlor 1254 strongly supports the earthworm's suitability as a model organism for analyzing xenobiotics. The purpose of the proposed research is to extend that work by examining in detail the complete immune system of the earthworm using Arochlor 1254 to identify the most efficacious immunoassay tools for screening xenobiotic chemicals and understanding the mechanisms of their immunosuppression. To that end a set of experiments is designed using Arochlor 1254 to assess the integrative effects of an environmental xenobiotic, with known immunousuppressive potential on cell-mediated and humoral components of the earthworm's immune system. The scientific community will benefit from an increased understanding of how xenobiotics affect various immune responses.